This invention relates generally to the fields of optical modulators, light valves, electro-optic filters, projection and flat panel display devices. More specifically, this invention relates to an optical modulating device, light-valve display or optical filter which uses a micro-dynamic construction to exploit color-selective absorption at a metal-dielectric interface by surface plasmons.
Yu Wang (Ref 1) has reported on the phenomenon of voltage-induced color-selective absorption at a metal/liquid crystal interface with surface plasmons. The surface plasmon, a collective excitation of electrons, absorbs all incident light at the resonance frequency of the plasmon. When incident p-polarized light is absorbed at the surface plasmon resonance, the reflected light will show a color complementary to that which is absorbed.
Wang teaches that by using a liquid crystal, whose dielectric constant varies with applied voltage, one can change the resonance frequency of the surface plasmon which in turn provides a concomitant change in absorption. Changing the dielectric constant of the liquid crystal through the application of voltage results in the reflected light showing a color change.
FIG. 1 schematically shows the prior art as taught by Wang. In this figure, the projection display 10 includes a substrate 11, bottom electrode 12, alignment layers 13, spacers 14, top electrode 15, liquid crystal 16 and seal 17. Alignment layers 13, typically formed by deposition of an oxide or polyimide layer and mechanical rubbing of the surface, are required to be be employed to impart a preferred direction to the liquid crystal 16. Spacers 14 are required to set the desired spacing between top electrode 15 and bottom electrode 12. Seal 17 is required to contain liquid crystal 16 between top electrode 15 and bottom electrode 12.
In U.S. Pat. No. 5,570,139 Wang describes a similarly designed liquid crystal embodiment and how this embodiment may be employed in projection display applications. Also briefly described is the use of a solid state substitute for the liquid crystal, however a detailed design of the solid state approach is not presented.
In U.S. Pat. No. 5,986,808, Wang describes a surface plasmon tunable filter using metallic layers bordering a dielectric region with an adjustable air gap as a dielectric. Wang teaches the use of piezoelectric spacers to physically expand or contract the air gap when a voltage is applied.
Similarly, Wang, in U.S. Pat. No. 6,031,653, describes thin-film metal interference filters forming a Fabry-Perot cavity in which piezoelectric spacers are used to control an air gap between two metal films.
The complexities of liquid crystal and incorporated piezoelectric structures pose substantial limitations towards the ability to rapidly and easily manufacture optical modulating devices in great numbers and at once.
A need exists for optical modulating devices that are compatible for monolithic integration with silicon, silicon-germanium, silicon-on-sapphire (SOS) or silicon-on-quartz (SOQ) advanced microelectronic technology (NMOS, PMOS and CMOS) for integrated control circuitry, electrical addressing, system interfacing, and the like so that these may be rapidly and greatly manufactured.
The invention is a optical modulating device capable of being used as a light-valve, display, optical filter or Fabry-Perot cavity, for example. The optical device of the invention lends itself to batch processing, and is compatible with monolithic integration with silicon, silicon-germanium, silicon-on-sapphire (SOS) or silicon-on-quartz (SOQ) advanced microelectronic technology (NMOS, PMOS and CMOS) for integrated control circuitry, electrical addressing, system interfacing, and the like. The device forgoes the complexities of liquid crystal constructions, and avoids the need to position and fix piezoelectric spacers within layers of the device.
In this invention, first and second transparent layers are disposed to oppose one another. Metallic layers are disposed upon the inwardly facing surfaces of the transparent layers and these are arranged to oppose each other. Spectral coupling layers are disposed upon the outwardly facing surfaces of the transparent layers opposite of the metallic layers.
In one embodiment, electrodes are disposed upon the inwardly facing surface of the first and second transparent layers. An optical cavity is created between the metallic layers, and voltage is applied to the opposing electrodes to adjust the air gap between the metallic layers. Adjustment of this air gap permits light passing therethrough to be controlled.
In a second embodiment, electrodes are omitted, and the air gap within the optical cavity is adjusted via the application of heat to the optical device. Light is thereby controlled through the selective expansion and contraction of the air gap.
In a preferred approach to both embodiments, the metallic layers are of the same thickness and material. A surface plasmon effect is created at the interfaces of the air dielectric and metallic layers. In a well understood manner, the surface plasmon effect serves to selectively filter light transmitted through the device, so that a desired light transmission results at the output of the device.
In batch processing fabrication, the optical device of the invention may be manufactured in halves that each include a transparent layer, a metallic layer, a spectral coupling layer and an electrode, when used. The halves are later aligned in opposing couples. In this fabrication scheme, the halves may be produced in substantially the same plane.
An alternate processing design includes fabrication of the layers above, but in an opposing fashion, so that the air gap is later created such as by etching.
To facilitate movement of one transparent layer with respect to another, one or more flexible portions are incorporated into at least one of the layers. This design lends itself to batch processing construction, and avoids the aforementioned need to align and fix piezoelectric spacers such as those used in prior art inventions.
The design also avoids the requirements of liquid crystal configurations, such as seals, alignment layers and the like.
It is an object of this invention to provide an optical modulating device that lends itself to fabrication by batch processing.
A further object of this invention is to provide an optical modulating device that eliminates the use of liquid crystals.
Yet another object of this invention is to provide an optical modulating device of relatively high temperature resistance and/or operating range.
Yet a further object of this invention is to provide an optical modulating device that provides the ability to modulate high levels of light flux or fluence, such as from a laser source.
A further object of this invention is to provide an optical modulating device that is compatible for monolithic integration with silicon, silicon-germanium, silicon-on-sapphire (SOS) or silicon-on-quartz (SOQ) advanced microelectronic technology (NMOS, PMOS and CMOS).
Other objects, advantages, and new features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanied drawings.